Influence of He++ and Shock Geometry on Interplanetary Shocks in the Solar Wind: 2D Hybrid Simulations

摘要

After protons, alpha particles (He~(++)) are the most important ion species in the solar wind, constituting typically about 5% of the total ion number density. Due to their different charge-to-mass ratio, protons and He~(++) particles are accelerated differently when they cross the electrostatic potential in a collisionless shock. This behavior can produce changes in the velocity distribution function (VDF) for both species generating anisotropy in the temperature, which is considered to be the energy source for various phenomena such as ion cyclotron and mirror mode waves. How these changes in temperature anisotropy and shock structure depend on the percentage of He~(++) particles and the geometry of the shock is not completely understood. In this paper, we have performed various 2D local hybrid simulations (particle ions, massless fluid electrons) with similar characteristics (e.g., Mach number) to interplanetary shocks for both quasi-parallel and quasi-perpendicular geometries self-consistently including different percentages of He++ particles.We have found changes in the shock transition behavior as well as in the temperature anisotropy as functions of both the shock geometry and He~(++) particle abundance: The change of the initial θBn leads to variations of the efficiency with which particles can escape to the upstream region facilitating or not the formation of compressive structures in the magnetic field that will produce increments in perpendicular temperature. The regions where both temperature anisotropy and compressive fluctuations appear tend to be more extended and reach higher values as the He~(++) content in the simulations increases.